In energy industries, fluid testing is an important component in evaluation of formations and oil and gas production. Such testing can be useful in evaluating the hydrocarbon content of a formation or drilling fluid, and can be performed downhole, e.g., during drilling. Internal reflectance spectroscopy is a process that is useful for evaluating formation fluids. Internal reflectance spectroscopy utilizes a transparent crystal having a surface in contact with a fluid. Internal reflections of a light beam inside the crystal from the crystal-fluid interface are evaluated to estimate fluid properties.